Atmosphere is a mod designed to play a short piece of music on entering a zone. It's designed to give travelling a better atmosphere to it. For example, if you were to set foot back into Thunder Bluff, then perhaps the piece of music being played would be welcoming, or tribal. If you were to set hoof into the Searing Gorge, perhaps the music would be all dark and ominous. If you decide to use this you'll have to build up your own database of ditties, but having worked on such myself, I think it's worth it and it makes trotting around a little more interesting.
How do I use it?
Drop some music files in the Media folder to start out, keeping in mind which you want to use for each zone (you can't change the file-structure after Warcraft starts, it's a hard-coded Blizz limitation). Once in game, go to the zone you want to setup and then type /atmos (that is indeed a nod to Doctor Who) or go into the main menu, then Interface, then Addons, then Atmosphere.
On the config screen, you have three options. In one you can enter the name of a music file, the one you wish to hear. You can also limit the length of the file, or allow it to play more than once (whatever you desire). And finally there's an option to reset those settings.
To setup Atmosphere, simply head to each zone you wish to have musical interludes in and set those options.
1.0: I feel this mod has reached a milestone, in that it's achieved a level of stability to the point where (despite my efforts to break it) I haven't seen any bugs in quite some time. This milestone also implies that I'm unable to think of any relevant features to add at this time without bloating out this mod, and there's nothing left to do that wouldn't be superfluous. Thus, this mod will likely be left untouched until a new feature is desired, or until a new Warcraft patch breaks it (if that happens). As of now, this mod should be considered stable.
revision170508b: I knew I'd forgotten something, and it struck as soon as I entered combat. I've now thrown in some checks so that Atmosphere won't play music on changing zones if the player is in combat. This is so that there'll be no distractions in combat, and so that it won't break Epica (also good).
revision170508: Complete rewrite, Atmosphere is now much more friendly.